What is the most common cause of obstructive sleep apnea in a 4-year-old child?

In young children, the most common cause of obstructive sleep apnea (OSA) is adenotonsillar hypertrophy (enlarged adenoids and tonsils). The relatively large lymphoid tissue in proportion to the upper airway narrows the pharyngeal space during sleep. This is why adenotonsillectomy is the first-line surgical treatment for pediatric OSA. Allergic rhinitis contributes to nasal obstruction but is not the primary cause. Micrognathia can predispose to OSA but is less common. Asthma is a lower airway disease and not directly associated with OSA.